the elements determined, in spite of substantial variations between bulk Allende and that meteorite's spheroidal Ca-Al-rich inclusions (Chou et al., 1976). ACKNOWLEDGMENTS.-We are indebted to R. Bild, R. Glimp, J. Kimberlin, K. Robinson, and L. Sundberg for assistance. Neutron irradiations at the UCLA and Ames Laboratories reactors were handled by J. Brower and A.F. Voight and their associates. This work was supported by NSF Grant GA 32084. 1450±40 c , 1400 e , 1300 f , l700 g 3370±100 c , 3300±100 d , 3300', 3000 f 200 e , 250 f , 1800±1200 g 130±22 c , 70 e , l70 f 3680±100 c , 3900±100 d , 3600 e , 4200 f , 4200 g 640±20 c , 650±40 d , 600 e , 700 f , 600 g 11.0±0.5 c , 12.2±0.2 d , ll e , 10 g 0.027±0.001 c 0.19±0.01 c 3.0±0.1 c , 2 e 0.44±0.02 c , 0.7 e 1.25±0.06 c , l e 0.20±0.01 c , 0.2 e 0.91±0.05 c , 0.9 e 0.29±0.01 c , 0.5 e 0.107±0.005 c ,0.1 e 0.43±0.02 c , 0.6 e 0.074±0.005 c , 0.09 e 0.42±0.02 c , 0.6 e 0.12±0.1 c ,0.1 e 0.31±0.02 c , 0.3 e 0.049±0.001 c 0.32±0.02 c , 0.4 e 0.058±0.002 c * The In value determined for 10-2 is 0.02±0.01; the large error limit is imposed due to an analyzer malfunction during counting. b The Cd value for 12-2 could not be determined because of a laboratory accident during the chemical processing.

No abstract

— Watson, which was found in 1972 in South Australia, contains the largest single silicate rock mass seen in any known iron meteorite. A comprehensive study has been completed on this unusual meteorite: petrography, metallography, analyses of the silicate inclusion (whole rock chemical analysis, INAA, RNAA, noble gases, and oxygen isotope analysis) and mineral compositions (by electron microprobe and ion microprobe). The whole rock has a composition of an H‐chondrite minus the normal H‐group metal and troilite content. The oxygen isotope composition is that of the silicates in the HE iron meteorites and lies along an oxygen isotope fractionation line with the H‐group chondrites. Trace elements in the metal confirm Watson is a new HE iron. Whole rock Watson silicate shows an enrichment in K and P (each ∼2X H‐chondrites). The silicate inclusion has a highly equilibrated igneous (peridotite‐like) texture with olivine largely poikilitic within low‐Ca pyroxene: olivine (Fa20), opx (Fs17Wo3), capx (Fs9Wo41) (with very fine exsolution lamellae), antiperthite feldspar (An1–Or5) with <1 μm exsolution lamellae (An1–3Or>40), shocked feldspar with altered stoichiometry, minor whitlockite (also a poorly characterized interstitial phosphate‐rich phase) and chromite, and only traces of metal and troilite. The individual silicate minerals have normal chondritic REE patterns, but whitlockite has a remarkable REE pattern. It is very enriched in light REE (La is 720X C1, and Lu is 90X C1, as opposed to usual chonditic values of ∼300X and 100–150X, respectively) with a negative Eu anomaly. The enrichment of whole rock K is expressed both in an unusually high mean modal Or content of the feldspar, Or13, and in the presence of antiperthite. Whole rock trace element data for the silicate mass support the petrography. Watson silicate was an H‐chondrite engulfed by metal and melted at > 1550 °C. A flat refractory lithophile and flat REE pattern (at ∼1x average H‐chondrites) indicate that melting took place in a relatively closed system. Immiscible metal and sulfide were occluded into the surrounding metal host. Below 1100 °C, the average cooling rate is estimated to have been ∼1000 °C/Ma; Widmanstätten structure formed, any igneous zoning in the silicates was equilibrated, and feldspar and pyroxene exsolution took place. Cooling to below 300 °C was completed by 3.5 Ga B. P. At 8 Ma, a shock event took place causing some severe metal deformation and forming local melt pockets of schreibersite/metal. This event likely caused the release of Watson into interplanetary space. The time of this event, 8Ma, corresponds to the peak frequency of exposure ages of the H‐chondrites. This further confirms the link between HE irons and the H‐chondrites, a relationship already indicated by their common oxygen isotope source. Watson metal structures are very similar to those in Kodaikanal. Watson, Kodaikanal and Netschaëvo form the young group of HE meteorites (ages 3.7 ± 0.2 Ga). They appear to represent steps in a chain of events that must have...

Schreibersite growth and its influence on the metallography of coarse-structured iron meteorites.